


Antithesis

by Kimber_Prime



Series: UnderFall [1]
Category: Undertale (Video Game)
Genre: Corruption, Death, Loss, Other, Sadness
Language: English
Status: Completed
Published: 2016-06-17
Updated: 2016-06-18
Packaged: 2018-07-15 09:15:00
Rating: General Audiences
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 9
Words: 7,566
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/7216519
Author URL: https://archiveofourown.org/users/Kimber_Prime/pseuds/Kimber_Prime
Summary: <blockquote class="userstuff">
              <p>Just another day in the Underground suddenly turns into Sans's worst nightmare when a mysterious corridoor appears in Waterfall.</p>
            </blockquote>





	1. The Forsaken One

**Author's Note:**

> This story does not follow the original game completely, so facts may not be the same.
> 
> Some changed include (they may not all stay this way for the whole story!):  
> \- Sans has more HP.  
> \- Only a few humans have fallen down so far, and none have been genocidal.  
> \- Undyne has both her eyes.  
> \- Papyrus and Sans are closer together in age.

He inhaled deeply walking through the echo flowers; taking in the smell of moist grass and dirt. Skeletons didn’t need to breathe of course, but that doesn’t mean they don’t enjoy it.

Sans had been walking through Waterfall for a while now, on his way home after visiting Alphys on some unofficial business; taking his shortcuts would have been much faster than walking all the way back to Snowdin, but there was no rush so Sans had decided to take his time and see the sights.

He walked past his outpost, briefly stopping to make sure everything was still in place for the next time he had sentry duty there. Papyrus wouldn't be happy if he came past here, possibly to visit Undyne, and found it in shambles. Satisfied, he continued on to where his telescope stood.

“Hehe...” he chuckled to himself, remembering all the times he’d tricked monsters by putting red paint on the eye piece. No one fell for that anymore these days, but those had been some good times.

Walking further a nagging feeling slowly began to emerge at the back of his mind. Something wasn’t right here, but what? At that moment he realised that he was walking through a corridor that shouldn’t be there.

The area his telescope was in should lead straight to the room with the echo flower and the cheese crystal, so why was this suddenly here? Looking back he couldn’t even see the previous room anymore.

He kept walking, expecting the corridor to end at any moment. When it didn’t he started running, but there was still no room in sight.

Coming to a halt Sans looked around. “What...?” In both directions it seemed to be a never ending corridor.

“Alright then”. 

Summoning up his magic Sans tried teleporting to Snowdin, but as soon as he called upon his magic he knew something was wrong; the feeling he usually had while using his powers replaced by something odd and foreign.

“Omph” he exclaimed as he hit the ground face first. “What now...?”

Sitting up he could see he was still in the seemingly never ending corridor but one thing was blatantly different: a grey door stood before him.

Sans got to his feet, examining the door in confusion. He’d never seen a door like this in Waterfall before, but then again he didn’t know if he was even still there.

He felt really uncomfortable about the whole situation, but with no end to the corridor in sight he didn’t really think he had another choice.

Taking hold of the handle Sans carefully opened the door, a mist pouring out of it as soon as it was ajar.

Inside everything was pitch black and Sans couldn’t tell where the floor, ceiling or walls were, if they were even there at all.

Carefully stepping forward he could feel his feet touching solid ground. “Ok, check on the floor” he mumbled, taking another look around. 

On closer inspection Sans realized that there was something in the room that wasn’t all black; the shape of a man was sitting further out into the room, unmoving, seemingly unaware of him being there.

“Hey buddy, you know how to get out of this place?” Sans called out while walking closer. There was no response from the figure.

“Welp, not the talkative type eh? Well I-“ he suddenly stopped as he got a better look of the person in front of him.

A skeleton dressed all in black, cracks in his head and holes in his hands; someone Sans instantly recognized.

“Gaster?!” he exclaimed, running closer. Was Gaster ignoring him? No, surely he wouldn’t.

“Gaster? Can you hear me?” Gaster didn’t seem to hear him, or if he did he showed no indication of it.

“Hey, why aren’t you answering?” Sans asked reaching out a hand.

As soon as he touched the former scientist he knew that he’d made a mistake; magic rushed into his hand and arm, spreading rapidly throughout his body and enveloping his soul in a burning sensation. The worst part was that no matter how hard he tried to struggle he could not seem to remove his hand from the other’s shoulder.

“G-Gaster?!” he stammered through the pain, collapsing to his knees and looking up at the scientist.

Gaster’s head lifted for a moment to look right into his eyes. Sans could clearly see the sadness in them, a black liquid dripping from one eye socket.

‘I’m sorry’ suddenly echoed in his head as Gaster scattered into dust.

Sans was now free but the pain continued, seeming to gather at his soul; a sensation he couldn’t describe in any other way than being burned from the inside out.

It was agony yet somehow it started to become even worse. He screamed, desperate for the pain to stop, only for it to keep increasing. 

Eventually it became too extreme and Sans blacked out, a blissful darkness overcoming him.


	2. Torment

Opening his eyes a long groan escaped him. The pain in his soul was gone but he still felt weird and his skull was hurting as if someone had just tried to crush it.

Sitting up he could see he was once again in Waterfall but now by the cheese encased in a crystal. He was no longer in the infinite hallway or the dark room with-

“Gaster!” Sans suddenly exclaimed and looked around but there was no sign of the scientist anywhere, even the corridor seemed to have disappeared.

He stared back towards the room with his telescope for a while but when nothing happened he picked himself up off the ground. He was quite dizzy and felt weak but knew he had to get back to Papyrus. 

Sans didn’t know how long he had been knocked out and it was impossible to tell down here since there was no day and night cycle. He had to get to Snowdin before his brother got too worried.

Luckily it wasn’t too far anymore and Sans felt like he was strong enough to make it on foot; no way he was going to try teleport at the moment.

He stumbled repeatedly and had to stop once on the outskirts of Snowdin for a quick rest, but eventually he made it home.

“SANS, THERE YOU ARE!” Papyrus exclaimed from the kitchen after Sans opened the door. “DID YOU FALL ASLEEP AT ALPHY’S HOUSE?! IT’S ALREADY DINNER TIME!”

“Uh, yeah. Sorry about that Papyrus. Heh, guess I was bone tired from all the walking I did today.”

Papyrus groaned so loud that Sans could hear it from the couch he had taken a seat on since entering. Leaning back Sans closed his eyes for a moment. His head was still throbbing slightly and there was a strange sensation in his soul, but he would look at that later once he was alone. He didn’t want to worry his bro.

Speaking of Papyrus, Sans’s thoughts were interrupted by the sound of him entering the room. Opening his eyes he could see Papyrus in his chef’s hat and apron which read ‘Unparalleled Spaghettore’ carrying two plates of Spaghetti towards the dining table.

Getting up off the couch Sans joined his brother at the table, quickly digging into the slightly undercooked spaghetti. He didn’t realize how hungry he had been, and even went to get a second plate from the kitchen after finishing the first.

“WOWIE SANS! I’VE NEVER SEEN YOU EAT THIS MUCH BEFORE! ALL THAT WALKING MUST HAVE REALLY BUILT UP AN APPETITE!”

Sans simply nodded as he finished his plate, resting his head on the table afterwards, exhausted. 

“YOU SEEM MORE TIRED THAN USUAL SANS. WHY DON’T YOU GO LIE DOWN WHILE I CLEAN THE DISHES?”

He nodded sleepily before picking himself off the table and heading upstairs, collapsing on his bed when he reached it and letting sleep claim him.

It was peaceful in his dreams, silence all around as he drifted. It did not last for long.

“NOOOO!” Sans heard someone emit a bloodthirsty cry and his eyes flew open. He suddenly realized that the cry was coming from him and he quickly silenced his voice. Snow surrounded him on all sides, but what his eyes were instantly drawn to was the bright orange object on the ground before him.

It was Papyrus’s scarf, lying in a pile of dust.

He screamed again and bolted upright, once again back in his bed with his sheets discarded and a blue glow filling the room, but also with vivid visions playing out before him.

“SANS!” he heard Papyrus call out as they rushed into his room, but he couldn’t focus on Papyrus; the burning in his soul had returned once again.

“SANS SPEAK TO ME!” his brother was right up in front of him now, looking into Sans’s eyes with great worry, but he was not looking back.

Tightly holding his sides Sans was trying to calm himself down and make the pain subside while more images flickered in front of his face; people he knew about to die, friends trying to kill him, what scared him most of all though were the images of Papyrus.

They were gruesome and horrible visions of him being killed repeatedly by people both he and Sans knew; Undyne, Asgore, even Alphys, all striking down Papyrus without mercy. Even worse was the taunting that followed; how he should have saved his brother, that he was a failure and didn’t love Papyrus at all.

Sans begged for them to stop, but they kept on going relentlessly, either not hearing or blatantly ignoring his pleas. The voices kept calling him out, over and over, pushing him to his breaking limit until he noticed a different voice speaking to him; one so quiet that it was almost drowned out by the rest but unlike the others this one was kind, whispering words of encouragement.

He focused on the voice as best he could, hanging onto it as if it was a lifeline.

‘Hang on Sans’  
‘Don’t listen to them’  
‘Focus on Papyrus’s voice’

As if he had somehow heard the strange voice in Sans's head, his brother's voice suddenly started breaking through; slowly silencing the others until they completely faded.

“SANS! SANS PLEASE LOOK AT ME! SANS!”

Snapping his head upwards Sans was suddenly face to face with Papyrus, tears streaming from both skeleton’s sockets.

His brother was alive. Papyrus was alive. 

“P-Papyrus?” Sans asked quietly.

“I’M HERE BROTHER! IT’S ALRIGHT! YOU’RE SAFE!” Papyrus replied grabbing his brother into a tight embrace. Sans quickly returned it, taking comfort from the other’s presence.

“SANS, WHAT HAPPENED?! I HEARD YOU SCREAMING AND WHEN I CAME IN YOUR EYES WERE GLAZED OVER WITH YOUR MAGIC NEARLY BURNING DOWN THE ROOM! THEN YOU STARTED YELLING, TELLING SOMETHING TO STOP AND GO AWAY!”

“J-just a bad nightmare bro. I’m alright” he felt far from it, but he couldn’t tell Papyrus what he had just seen.

“YOU ARE SHAKING LIKE A SNOWDOG IN THE WIND AND ALMOST SET THE ROOM ALIGHT, THAT DOES NOT SOUND ALRIGHT TO ME! YOU WILL COME SLEEP WITH ME TONIGHT! THAT WAY I CAN KEEP YOU SAFE!” 

Wrapping him up in his discarded blankets, Papyrus carried Sans back to his room where they got into his racecar bed together. Fortunately the bed was easily large enough for both of them.

“REST NOW BROTHER! I, THE GREAT PAPYRUS, WILL KEEP ALL THE BAD DREAMS AWAY!”

“T-Thanks bro” Sans replied in a shaky voice before lying down as close to Papyrus as he could manage. The shock of what had happened was slowly starting to fade, and exhaustion was taking its place once again.

Slowly sleep overcame him, taking him back to the empty darkness of his dreams.


	3. Amiss

The images haunted him again during his sleep but they were less severe than they had been before, the voice in his head keeping the worst of it at bay. Even so, he still felt exhausted when he awoke again.

Groggily sitting up Sans noticed that Papyrus was nowhere in sight, the clock reading 10am. Usually his bother would have woken him up by now but he was glad for the extra rest.

What had those dreams been about? Sure he’d gotten nightmares before but nothing like this. Did what happened with that weird door and Gaster have something to do with this? 

He still felt that strange feeling in his soul and was about to take a look at it when the door opened and Papyrus walked in.

“OH SANS YOU’RE AWAKE!” he exclaimed while quickly moving to be next to his brother. “HOW ARE YOU FEELING?”

“I’m alright Pap, I just can’t believe that you let my lazy bones sleep for that long” he said, giving Papyrus a grin and causing him to groan out loud. His expression soon became concerned though.

“I-I WAS WORRIED ABOUT YOU BROTHER. I’D NEVER SEEN YOU LIKE THAT BEFORE AND DIDN’T KNOW WHAT TO DO… I’M SO GLAD YOU’RE FEELING BETTER NOW!”

Sans reached out and grabbed Papyrus's arm, giving him a reassuring smile.

“ANYWAY, I HAVE BREAKFAST DOWNSTAIRS FOR YOU AND AM SUPPOSED TO GO MEET UP WITH UNDYNE AFTERWARDS FOR TRAINING, BUT PERHAPS IT WOULD BE BETTER IF I STAYED HOME WITH YOU?”

“Na Papyrus, don’t miss out on training because of me. How about I come with you instead? The dog guards were covering for you anyway so I’m sure they won’t mind covering for us both.”

“THAT IS A BRILLIANT IDEA SANS!” he exclaimed while suddenly embracing his brother, releasing him as he moved back towards the door. “NOW GET DRESSED AND COME DOWNSTAIR! WE HAVE A BUSY DAY AHEAD!”

“I’ll be right down Papyrus” Sans replied as Papyrus headed downstairs.

Getting up out of his brother’s bed Sans gathered up his blankets and went to his room. Quickly throwing his stuff onto the bed and donning his slippers and hoody he went downstairs, finding Papyrus at the table with breakfast spaghetti.

After finishing breakfast they washed the dishes together and headed off towards the river to try get a ride to Waterfall.

All of Snowdin seemed to be out and about with only a light shower of snow slowly falling; overall it was a lovely and peaceful day.

Everyone greeted the skeleton brothers as they walked past, wishing them a lovely day; a gesture they returned with a friendly wave before turning off towards the right and heading away from the townsfolk.

From up ahead a figure was headed their way, presumably running by how fast they were approaching.

Papyrus raised a hand in greeting as they got closer but the figure Sans could now identify as Monster Kid wasn't slowing down.

What he saw made his soul pulse in his chest; Monster Kid's eyes were burning red, a hateful smile on his face and his eyes fixed on his brother who was now looking the other way.

Emitting a hateful cry Monster Kid lept at Papyrus; feet first, claws outstretched.


	4. Phantom

Reacting instantly Sans created a wall of bones between Papyrus and the kid, quickly dragging his brother behind him.

What was wrong with them? Why were they attacking?

Summoning up more bones he prepared an attack when suddenly Papyrus ran back in front of him with his arms outstretched, blocking his view of the Monster kid.

"SANS STOP!" he shouted with confusion and fear in his eyes. "WHAT ARE YOU DOING?!"

Papyrus's reaction made Sans freeze; something wasn't right. Why was he protecting the thing which had just tried to attack him? 

Leaning to the side Sans looked at the kid he had trapped behind his wall; they looked completely ordinary, shaken and scared, but otherwise the same as normal. 

His attacks slowly dissipated as he stared at them in confusion and Papyrus quickly walked over to the young monster to quietly talk to them.

Sans didn't move and simply looked down at his hands; what had happened? One moment the kid had been charging Papyrus with murder in his eyes and the next he was standing there terrified of Sans attacking him. He didn't understand... 

Suddenly something was touching his shoulder, giving him a fright and making him snap his head up. Papyrus was before him, fear in his eyes.

"SANS? WHAT HAPPENED?"

"I-I don't know Papyrus..."

Monster Kid came into view behind Papyrus, hiding behind the skeleton best he could while still being able to see Sans.

"Y-yo, are y-you alright?"

Sans hesitated for a moment before answering, staring off into the distance as he did so and only briefly making eye contact with the kid. "N-no, I don't think so... Sorry about that kid..."

"THAT'S IT" Papyrus suddenly annouced. "WE'RE NOT GOING TO UNDYNE! WE ARE GOING BACK HOME WHERE I CAN KEEP AN EYE ON YOU! YOU BETTER HEAD HOME TOO MONSTER KID."

"A-alright, bye" he called as he ran off.

Sans was still staring straight ahead when Papyrus picked him up and started walking back into Snowdin.

"JUST REST SANS" he cooed, worry clear in his voice.

Heading into their house Papyrus gently lowered Sans onto the couch, wrapping him up in a nearby blanket before moving off a little way and calling Undyne. Sans wasn't paying much attention, but Papyrus seemed to be looking at his sockets a lot.

Once his brother had hung up he came over and sat down next to Sans.

"IT'S ALRIGHT SANS, WE'LL FIGURE THIS OUT! ALPHYS IS ALWAYS OUT OF PHONE RANGE AT THIS TIME OF DAY BUT I'LL RING HER UP AS SOON AS POSSIBLE. SHE'LL KNOW WHAT TO DO".

Sans just idly nodded while starting straight ahead at the turned off television. He couldn't seem to think or focus on anything, but he could see his reflection in the black screen; a stranger in his clothes staring back at him. His expression was dull and lifeless, but what stood out most were his sockets. No longer black with white pupils, or even completely black; instead they were grey, almost like fog had gathered in his skull. It seemed odd, but it hardly registered in his mind.

Suddenly he was tilted sideways; Papyrus gently laying him down across his lap and turning on the TV in an attempt to distract his brother.

"REST NOW SANS..." Papyrus said as Sans's reflection dissapeared into the featured quiz show.


	5. Inquiry

When Sans opened his eyes he was still lying on the couch, but Papyrus was no longer beneath him. His soul ached and everything was numb; even his mind seemed dull which made it hard to think.

He'd attacked Monster Kid, a child who would never hurt anyone. How had he let that happen?

Sitting up was a massive task for Sans with dizziness causing him to fall back repeatedly. Eventually he succedded and once upright he could hear Papyrus's voice coming from the kitchen, presumably on the phone with someone.

Not really caring if anyone saw anymore, he drew out his soul and looked at it. Usually resembling a shining blue heart, it was a dull blueish grey with tiny black cracks along the edges

Sans simply looked at it in shock and confusion; not knowing what it meant or how it had happened, although he assumed his encounter with Gaster had something to do with it.

Hearing Papyrus's voice getting louder he returned his soul and waited for his brother to exit the kitchen.

When he did his expression was worried with one glove held against his forehead, a phone in the other. As he noticed Sans though he quickly walked over and kneeled down in front of the couch.

"HOW ARE YOU FEELING SANS?" he asked quietly, or as quietly as he could manage with his loud voice.

"Pretty horrible to be honest..." Sans replied, his voice sounding like he hadn't spoken in years. "Can't believe I almost hurt that poor kid..."

"YOU'RE SICK SANS, IT'S NOT YOUR FAULT. YOU WERE ONLY TRYING TO PROTECT ME"

He sighed quietly before continuing.

"I JUST CALLED ALPHYS, SHE'S HOME NOW AND CAN SEE US WHEN WE'RE READY. I WAS ABOUT TO COME WAKE YOU WHEN I SAW YOU SITTING UP".

"Hate to ask, but I don't think I can walk that far right now... Could you-?"

"YOU NEVER HAVE TO ASK ME THAT QUESTION SANS. OF COURSE I WILL. JUST LET ME GRAB A DIFFERENT BLANKET"

While Papyrus walked upstairs Sans looked back towards the television; it was once again turned off and he could see his sockets were still the strange grey colour. For a second he also thought his clothes had changed colour, but when he looked again it was simply his blue hoody.

Papyrus returned carrying a dark blue blanket he'd take from Sans's room and wrapped it gently around his brother, picking him up and carrying him outside straight after.

They headed towards the river, finding the River Person waiting there once they arrived. Somehow they always seemed to know where there would be people wanting a ride and would be ready for them once they got to the river.

Papyrus greeted the River Person, earning a nod in return, before both brothers embarked and they set off for the Hotlands.

It didn’t take long to get to their destination. Thanking the River Person as they got off Papyrus headed towards Alphy's lab with Sans in his arms; luckily it wasn't far and soon the skeleton brother was knocking on the lab door.

"C-coming" a voice said from within and the door opened shortly after, revealing Alphys on the other side. "C-come on in".

They walked in silence through the lab until they reached a room with a large machine in the center of it. It was a giant cylinder with a platform sticking out of it and multiple scanners inside of it.

"P-put him on there Papyrus"

Gently unwrapping Sans he complied and placed his brother into the machine on his back, the platform moving inside it as Alphys pressed a button on the wall.

"W-we have to leave the room for this Papyrus. I-it shouldn't take l-long though".

Briefly placing his hand on Sans's leg, the only part of his brother he could reach, he headed towards the door. "WE'LL BE RIGHT BACK SANS".

Sans simply stared at the devices above him after hearing the door close. Alphys would be able to help, he hoped.

As the machine powered up and the lights came on he simply closed his eyes, sleep claiming him once again.


	6. Dread

Alphys ran several tests on Sans while he drifted in and out of consciousness, Papyrus always near his brother's side when Alphys allowed it.

At one point Sans was also given some sort of medication which initially caused him to fall asleep but he felt stronger and slightly more alert afterwards. 

Nearing late afternoon Alphys finally finished her tests and approached the two brothers who were seated in the hallway; Sans was rubbing his sockets repeatedly while trying to clear his vision that had started to become blurry over the past few hours.

"A-all the tests are now done. I s-should hopefully have results by tomorrow b-but in the meantime you need to rest Sans, and k-keep taking that medication every two hours".

"Thanks Al" he said quietly, his voice still sounding weird.

"LET'S GO HOME SANS. THANKS FOR EVERYTHING ALPHYS".

"Y-you're welcome. I'll get started analysing the results r-right away".

"Thanks"

Sans was walking on his own now as they headed towards the exit and said goodbye to the scientist. Once outside he froze while watching Alphys head back to her lab; had she been wearing grey for a second? No, Sans thought to himself, it must just have been my imagination.

"Say Papyrus, do you want to go to Undyne?" Sans suddenly asked looking up at his brother. "I'm sure Undyne won't mind me crashing there and it doesn't matter where I rest. Besides it's my fault you didn't get to go to see her yesterday".

"ARE YOU SURE SANS? WOULDN'T YOU RATHER REST AT HOME?"

"You know I can fall asleep anywhere bro, it doesn't bother me" he replied with a slight grin.

"WELL... ALRIGHT THEN. BUT WE WON'T STAY THERE FOR THAT LONG, AND IF ANYTHING FEELS STRANGE YOU TELL ME SO THAT WE CAN HEAD STRAIGHT HOME! NO DISCUSSIONS!"

"Sounds good to me"

As they reached the river they asked the Riverperson to take them to Waterfall and set off. The ride was peaceful although Sans could not stop staring at the cloaked figure; he was convinced that their eyes had flickered to blue and orange for a moment, but it had vanished just as fast as it had appeared. Disembarking on the shores shortly after, the brothers set off towards Undyne.

The fish shaped house soon came into view and Papyrus quickly went to knock on the door, which opened shortly after.

“Oh, hi Papyrus” Undyne said when she saw the skeleton in front of her door. "What are you doing here? You said Sans was sick.”

“I am” Sans replied from behind Papyrus "but Alphys told me I just needed to rest while she analyses my test results. I didn't want to make Papyrus miss out on this because of me".

“If you're sure... It's your choice. There's a lounge chair right here by the door you can use and watch us from if you'd like.” she replied with a slightly worried expression. “Anyway, let’s get started Papyrus. We’ll begin with the training dummy today”.

Sans sat down in the chair and leant back into it while Papyrus practiced hitting the dummy with his bone attacks. He wasn’t really paying attention though, his thoughts drifting back to the previous night again. With the medication Alphys had given him his mind was resonably clear once again.

He pondered it for what seemed like hours, but he couldn’t come up with answers. Gaster died years ago, but somehow Sans had seen him anyway? What had happened to him when he had touched the scientist? 

Sans briefly looked up, seeing that Papyrus and Undyne had move onto sparring before he started thinking again. 

The voices were still there at the back of his mind, a faint whisper. He really hoped that Alphys could find a way to make them stop before he completely lost his mind.

Papyrus suddenly grunted aloud and looking up Sans was greated by a sight that filled him with horror.

His brother was staggering back, a large gash in his torso, and before him stood a human; a freaky grin on their face as their eyes shone in the colour of blood, a knife in their hand.

Sans reacted on instinct, firing bones and blasters towards the human trying to kill his brother. The human tried to block and dodge his attacks but he refused to relent, sending one attack after the other until eventually the area was filled with dust from the fight.

Letting it clear he prepared to fire once again, but what he saw made him freeze; before him was Undyne, clearly injured with Papyrus lying beside her after having been knocked back by the explosions. He was further uninjured, and there was no human. 

Papyrus was now looking up at him, shock clearly reflected on his face as he stared at his brother. 

"S-SANS...?" he asked in a shaky voice while Undyne was slowly climbing to her feet, blood dripping from her hands.

Through the shock of what had just happened Sans fled with tears in his eyes, knowing only that he had to get away from those he cared about before he hurt anyone else.


	7. Misconception

Sans ran through Waterfall, not caring where he was going as long as it was as far away from Papyrus and Undyne as he could get.

First he'd almost hurt the kid, now Undyne was injured because of him. He really was losing his mind; the voices and burning in his soul returning once again. He was terrified as he kept going blindly forward with tears streaming down his face until eventually he stumbled and fell, faceplanting into snow.

He'd run back towards Snowdin it seemed with the small town appearing before him as he lifted his head. Picking himself up he quickly kept going.

His vision still hadn't returned to normal but he could recognise their house well enough when it came into view; going inside and shutting the door behind him he collapsed to the floor. He was in shock and taking deep, heaving breaths to try calm down even though he knew it was pointless; he'd hurt one of his best friends. What was going to stop him from hurting someone else, or possibly even killing them?

Papyrus... He had to stay away from him, so staying here was not an option. Getting up he was about to head back out the door when he saw something that made him stop; there was a mirror off to the side, and he couldn't help but stare at his reflection.

His sockets were still grey as before but the colour had also leeched from his clothing, leaving it grey and dull instead of the usual brighter colours.

The sight angered him; what had he done to deserve this he screamed internally before smashing the mirror with all his might. The shattering glass scatted over the ground and took off some of his HP but he didn't care; running out the door straight after and dissapearing into the snow, the voices taunting him as he went.

Heading towards the ruins he made a sharp left off the path and into the forest, panting as he kept running to increase the distance between him and Papyrus. Eventually he couldn't take it anymore and collapsed face first into the snow, sobbing until he blacked out.

When he awoke again it had started snowing and he had already become partially buried. Getting up shakily he continued onwards, away from Snowdin. He wasn't sure how long he had been lying there and he did not want Papyrus to catch up.

His brother... They loved each other so much yet he was running away from him for his own protection. Sans knew Papyrus wouldn't blame him for what happened to Undyne, but that expression that had been in his eyes; it had been one of fear.

He couldn't risk hurting his brother, even if it meant never seeing him again. With tears in his eyes he continued on until he reached a small clearing and someone began appearing from the gloom ahead.

Seeing the figure Sans wanted to turn away and quickly dissapear into the trees, scared that it might be Papyrus, but he could soon see that this wasn't the case.

What he saw was possibly the only person he wanted to see less than Papyrus right now: Gaster.

How he was there Sans didn't know, but the hate he felt towards the scientist right now was undescribable; he wanted nothing more than to destroy him for what he had done. Somewhere at the back of his mind he could feel himself scream out to not do this, but he didn't care. All reason was lost to him.

"Sans" the figure before him began to speak in a mocking voice. "Aren't you happy to see me?"

"What did you do to me!" Sans yelled back at him, his magic ready to be unleashed at a seconds notice. "Why are you making me hurt my friends?!"

"I didn't do anything Sans. You were the ones who did it all".

"Liar! Tell me how to undo this!"

"First the kid, then Undyne. I wonder who you will attack next. Perhaps your brother?" he asked with a menacing grin. "Sans... I thought you were better than this..."

Sans unleashed everything he had on the apparition before him. He didn't care about anything but destroying him and making sure he was erased from existance for good. Gaster may have been a decent man in life, but there was no evidence of that in this thing.

Bones erupted from the ground and Blasters fired towards the scientist. Even when there was so much snow in the air that Sans could no longer see he kept firing. He was lost to himself now, no longer in control. The only objective in his clouded mind to kill, and the voice in his head that was begging him to stop silenced.

More attacks flew forward until finally Sans collapsed to his knees, his energy spent. He sat there panting in the flurry of snow which was upheaved during the battle swirling around him. A feeling of euphoria flowed through him at having blasted the scientist, a sensation forein to Sans but he reveled in it anyway.

Starting straight ahead as the snow began to settle he began to see something through the gloom where his target had been, but it was not Gaster.

It was Papyrus.


	8. Lapse

There was silence as Sans stood watching Papyrus in the clearing ahead of him, the larger skeleton's bones all broken and shattered with the remnants of his legs barely supporting him; his face reflecting horror and confusion as well as clear pain from both physical and emotional injuries of having his own brother attack him.

Sans blinked and let realisation wash over him. His thoughts cleared, the pain in his soul stopped, and the voices dissapeared. 

A feeling of alarm came over him as he stumbled backwards unsteadily, staring at what was left of his brother before him.

"S....SA...N...S..." Papyrus whispered in an agonizing voice before crumbling into dust right before his eyes, spreading over the snow. His scarf also floated to the ground, landing in the grey pile where he had stood moments before.

*You won! Your LOVE increased. 

"No... no no ... NO NO NO NO NO NO NO NO NO!!! PAPYRUS!!!" He screamed with tears streaming down his face. He couldn't move, he couldn't breath. He killed him... He had killed his own brother.

"I-I... T-this... No.... NO! This can't be happening, THIS CANT BE HAPPENING!!!"

Sans took another step away from what remained of Papyrus, tripping over the uneven ground and falling down. He clutched his chest, taking loud, pained breaths. The look on his face, it was haunting his mind.

"NO NO NO NO NO NO!!!" He screamed.

Sans couldn't take his eyes off his dust; the world spinning around him and his vision failing even worse. He couldn't seem to move, the throbbing in his soul returned, stronger then before, as well as the voices in his head. 

Suddenly a voice rang out calling Papyrus's name. Undyne had followed him even with her severe injuries and now appeared at the edge of the clearing; staring at the sight before her in horror while she clutched her left eye.

Despite the physical and emotional pain, Sans pushed himself off the ground and ran, not daring to look back, not even for a second; the only thought in his mind telling him to get away.

Sounds did reach him from behind though; the sound of pained screams and desperate cries as Undyne reached where Papyrus had been, sinking to the ground and sobbing her heart out over the loss of her best friend. 


	9. Epilogue

\--- One year later ---

Snow blew through the town in a horizontal blur, covering all of Snowdin in a thick white blanket. Yet despite these conditions someone was still walking through the storm; the most unlikely of canidates to be out at a time like this. Grillby.

It was early evening at the time as Grillby walked through the storm towards the Snowdin welcome sign, bag in hand. Once he reached it he kept walking until a hollow tree just off the main path came into view. Placing the bag inside he walked back towards the town.

As he arrived back at his bar he could see another figure out in the storm: Undyne was walking through Snowdin, shivering in the cold as she drew her coat tightly around her and tightly held onto the wreath in her hand, an eyepatch covering her left eye. At least the scarf she wore gave her some extra warmth.

Usually she avoided the snowy town as well as the heat of Hotland since she wasn't a fan of extreme temperatures but today she had a good reason to be here; to visit her friend.

Heading into the middle of town she soon stood before the house she'd come here to see. All the doors around her were decorated except this one so she'd made sure to bring a wreath with her today, especially since the inhabitants had loved to celebrate Christmas.

She missed the skeleton brothers, especially Papyrus. He'd been her best friend and hadn't deserved to die; he was innocent and nice to everyone. 

Hanging the wreath up on the door hook she went inside, heading upstairs as soon as she entered. The house had been empty since Papyrus's death and Sans's dissapearance so the locals had turned it into a memorial; each of the skeleton's possessions carefully laid out on their beds.

Sans's was pretty empty with only a jacket, a joke book and some ketchup on it but then again Sans wasn't actually dead, at least that's what almost everyone believed since no trace of him had been found.

Papyrus's on the other hand was stacked high with his possessions; his action figures, his flag, and his bone attacks just a few of the items. His dust rested in a decorative urn on top of the pile. The only item missing from the collection was his scarf which Undyne wore around her neck in memory of him.

Sitting down on the ground before her friend's bed she began to speak.

"Hey Papyrus, I know it's been a few weeks and I'm sorry for not coming back sooner; things have just been a bit crazy lately." "There's still no sign of Sans and the offical search has been called off, but I've taken some time off leading the Royal Guard to look for him; so far I've been unsuccessful."

She sighed before continuing. "I talked to Alphys about what was wrong with him and although she didn't know exactly what was wrong she was certain it wasn't his fault you..." 

"Anyway, something was wrong with his soul, something Alphys had never seen before. He should not even have been alive with the state it was in and it was giving off a strange energy."

"I don't know if I can find him or if I can even help him when I do, and I will, but I know that if you were here that you would never stop searching so I'm going to do that for you."

She lay a hand on the bed before her. "I promise you Papyrus that I will do everything I can to find him and bring him home, no matter how long it takes. I will get everyone to keep a lookout for him and I will personally keep searching as well."

Slowly standing up she walked back towards the stairs, stopping in the doorway and looking back.

"I'll see you again soon Papyrus... Merry Chrismas..." she whispered before heading downstairs and leaving the house behind her.

A figure watched her leave from the edge of the forest before they walked down into the town, bag in hand. He stayed within the shadows of the other buildings while looking across the street at the house Undyne had just left.

Sans had stayed around Snowdin for the past year, staying out of sight and simply watching on. He couldn't trust himself around others, not after what he had done.

Undyne didn't know what had happened to him, but during his year of isolation he'd managed to put together the pieces with the help of others. 

What most people didn't realise is that Gaster didn't die by accident, and Sans should know since he was working together with him at the time. Gaster was researching a theory about multiple timelines and something he liked to call the multiverse; that there were infinite worlds out there, all different in their own way.

On the day he died Gaster had been experimenting with opening a portal to one of these worlds to prove his theory, but something had gone wrong and the scientist dissapeared. Sans had searched for him of course, but to no avail. Somehow he must have ended up in that room Sans found him in: The Void. 

From previous research done by Gaster Sans had concluded that The Void was an empty blackness where nothing should exist; a desolate place between worlds in a state of limbo. Spending extended periods of time there would cause one's soul to become corrupted and eventually shatter.

Somehow a door had opened to The Void in Waterfall, how this had happened Sans did not know, but touching Gaster had caused the corruption to rapildy spread into him and destroy the scientist's soul, latching a part of it onto Sans's. It had been his voice that had tried to help him during the visions and voices that taunted him relentlessly and he still heard whispers of him today.

However since Sans was not trapped in the void and constantly exposed to the corruption his soul had not shattered like Gasters, something which could still happen in the future though. 

The visions and voices he'd been affected by also weren't simply delusions; they had been glimpses into these other realities, other timelines where those things had actually happened and he had begun to see more and more of them over time. 

Worlds where a human killed all monsters, ones where they set them all free, and even ones where monsters killed each other. There was no end to them and many were actually very peaceful, some even breathtakingly beautiful. 

It had been thanks to some lost souls he'd encountered that the pieces had finally come together; other assistants of Gaster's, trapped in some some alternate reality parallel to theirs. 'Gaster's followers' they called themselves and they were glad to have another monster to talk to. They'd helped him figure out what had happened.

For several months after what had happened to Papyrus Sans had felt nothing but hate for himself and sadness for his brother; in the end, he'd even given up on life. He'd lost hope, causing his HP to drop to just one point.

Firing his own attacks at himself in an attempt to end it he discovered that it had no effect on him; his pitiful 1 hp still there. In another desperate attempt he jumped off a cliff in Waterfall, smashing his legs as he hit the rocks but still he survived.

Unable to move he stayed trapped on the rocks for several days before one of Gaster's followers had found him: a young child who called themselves Goner Kid. They couldn't help him but they talked about what had happened, putting the first pieces of the puzzle into place.

Eventually someone else had found him. Grillby, on his way to buy snails at the Blooky farm he'd spotted Sans down below. Healing him with his magic and food on hand they talked briefly about what had happened; Grillby never pressuring him for answers and simply listening to what he wanted to share.

In the end after Sans had asked him to leave for his own safety Grillby had insisted that it wasn't his fault. He reasoned that if Papyrus was still alive that he wouldn't want his brother simply throwing his life away. Sans honestly didn't think that was true it but Grillby was the only person who still believed in him and therefore he agreed not to try end his own life anymore, especially since it didn't appear to work anyway.

Grillby still brings him food everyday, leaving a bag in Snowdin forest so he doesn't have to come into the town. He also never told anyone about it at Sans's request since if he was ever going to go back he wanted to do that himself.

After being rescued he'd gone to Hotland to talk to the other followers and visit what was left of Gaster's lab from the olden days, using the information from there to complete the puzzle.

He now knew what was wrong with him, his previous torment originating from some of the less pleasant alterate universes, but it seemed impossible to fix so he avoided everyone. Visions and voices still affected him every single day, and he was terrified of hurting someone else.

He had killed Papyrus with his own hands, a scene that played out before him in his dreams every single night, the voices in his head taunting him about it. He'd loved his brother more than anyone so if he hadn't been safe from him, who was?

Sighing Sans turned around and walked back into the forest, slowly fading into the snowstorm as he left Snowdin behind him.

He didn't believe what had happened could ever be fixed, forgotten, or forgiven, but as long as someone still believed in him he couldn't give up hope just yet.

\-------------

"In order to heal we must first forgive,  
and sometimes the person we must forgive  
is ourselves."  
\- Mila Bron

\-------------

END OF ANTITHESIS

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**Notes for the Chapter:**

> I hope you guys enjoyed reading Antithesis, and I am planning to write a sequel for it at some point in the future.
> 
> Inspiration for this story included:  
> "Animal I have become" and "Get out alive" by Three Days Grace  
> "Angels" by Vicetone  
> "Because of you" by Kelly Clarkson  
> "Monster" by Imagine Dragons and by Skillet
> 
> Thanks so much for reading everyone!


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